Sizing and engineering · Rolling Shutter Guide
Why do taller rolling shutters need bigger boxes?
The curtain rolls onto the axle in layers, and every extra wrap makes the roll fatter. A 38 mm insulated curtain 48 in tall rolls to about 5.45 in across; at 144 in it is about 8.67 in. The box has to hold the full roll, so a taller curtain steps up through the 6.5 to 12 in box sizes.
Key facts
- On a 60 mm axle, a 38 mm insulated curtain rolls to about 4.3 in across at 24 in tall, 7.2 in at 96 in and 8.7 in at 144 in. Roll-A-Shield data
- Each wrap of 38 mm insulated slat adds about 0.35 in to the roll's radius, a little less than the slat's 3/8 in thickness, because the slats nest as they wind. Roll-A-Shield data
- Across 27,349 shutters Roll-A-Shield has built, the 38 mm curtain came out at about the clear opening height plus 2.15 in, at 1.4713 in of curtain per slat. Roll-A-Shield data
- The same 8 in box holds a 38 mm insulated curtain up to 110 in tall but a 55 mm insulated curtain only up to 78-82 in. Roll-A-Shield data
Applies to: Rolling shutters of any make that roll onto a tubular axle inside a head box; the figures are for Roll-A-Shield 38 mm and 55 mm insulated and 63 mm single-wall slats.
When a rolling shutter opens, the curtain winds onto the axle inside the box like tape onto a reel. Every wrap adds a layer, so the roll gets fatter as more curtain winds on, and the box has to be big enough for the roll at its fattest, when the shutter is fully open. That is why box size follows curtain height and slat type, and only rarely width.
How big does the roll get?
For Roll-A-Shield's 38 mm insulated slat on a 60 mm axle, the rolled curtain measures roughly this across:
| Curtain height | Roll diameter, about | Smallest 38 mm box for that curtain | That box's actual height |
|---|---|---|---|
| 24 in | 4.28 in | 6.5 in | 6.575 in |
| 48 in | 5.45 in | 6.5 in | 6.575 in |
| 60 in | 5.95 in | 6.5 in | 6.575 in |
| 72 in | 6.41 in | 8 in | 8.150 in |
| 96 in | 7.24 in | 8 in | 8.150 in |
| 108 in | 7.63 in | 8 in | 8.150 in |
| 120 in | 7.99 in | 9 in | 9.055 in |
| 132 in | 8.34 in | 9 in | 9.055 in |
| 144 in | 8.67 in | 10 in | 10.039 in |
The roll diameters come from the slat maker's published roll sizes, fitted with a winding model that matches them to within about 2 mm on average. The box column is Roll-A-Shield's own chart, which reads the curtain height, the clear opening plus 2 in. Notice the roll does not grow evenly: the first 24 in of curtain adds a lot, the last 24 in much less, because each new wrap is longer than the one before it and so uses up more curtain.
How much does each wrap add?
About 0.35 in to the radius for the 38 mm insulated slat, or about 0.7 in to the diameter. That is a little less than the slat's 3/8 in thickness, because the slats nest into each other as they wind. The same model fitted to the 63 mm single-wall slat gives a thinner build per wrap, so that curtain takes more wraps for the same height.
Why do thicker slats need bigger boxes?
A thicker slat adds more to the roll with every wrap. The 55 mm insulated slat is 0.5625 in thick against 0.375 in for the 38 mm, and the box chart shows the difference:
| Box | 38 mm insulated, curtain up to | 55 mm insulated, curtain up to |
|---|---|---|
| 8 in | 110 in | 78-82 in |
| 9 in | 134 in | 99-114 in |
| 10 in | taller than 134 in | 124-150 in |
The 55 mm ranges depend on the axle: a 60 mm axle on lighter, narrower shutters, a 70 mm axle on heavier or wider ones, and the 70 mm axle takes the shorter curtain in each box. A bigger axle needs fewer turns to roll the same curtain, but its larger core makes the roll slightly fatter; for a 38 mm curtain 144 in tall the estimate is about 0.15 in more across on a 70 mm axle than on a 60 mm one. Full charts for every slat are in What size box do I need?.
How long is the curtain for my opening?
A little longer than the opening is tall. Roll-A-Shield's production records for 27,349 built shutters put the 38 mm curtain at about the clear opening height plus 2.15 in, with each slat adding 1.4713 in (37.37 mm) of curtain. The extra is the slat that stays in the rails below the box when the shutter is up, which is why the box chart reads the opening plus 2 in. Counting slats is covered in How many slats are in a rolling shutter?.
Can a box be too big?
A bigger box than the chart gives is allowed, but it takes more wall space and adds to the overall height, so check that it still fits. You can never have one smaller than the chart gives, because the rolled curtain would not fit. On a wide, heavy shutter the factory may step the box up one size anyway, when the axle would sag half an inch or more under the curtain; see Why wide shutters sag.
What next
Your quote gives the box size for each shutter. Send the clear opening sizes on a quote request, or call (480) 921-0200.
Frequently asked questions
Why does the 55 mm slat need a bigger box than the 38 mm at the same height?
It is a thicker slat, 9/16 in against 3/8 in, so each wrap adds more to the roll. The box chart reflects it: an 8 in box holds 110 in of 38 mm curtain but only 78-82 in of 55 mm.
Does a wider shutter need a bigger box?
Not for the roll itself, which depends on height and slat. A very wide, heavy shutter can still get the next box size up, because the axle bends slightly over a long span.
Parts and next step
Need a part or a hand?
Replacement motors, remotes, straps, slats and hardware ship from our Tempe, Arizona factory. Phoenix-area owners can book a service visit; everywhere else, our technicians help by phone. Call (480) 921-0200; the factory answers technical questions for any rolling shutter, not only ours.